You can see the mods you have subscribed to via the in-game modding hub - and unsubscribe (= uninstall) said mods from there. If the mod wasn't installed via the in-game modding hub or the Steam Workshop (= files just dropped manually in the game folder), you should re-download the mod, look at the files it installed / replaced, remove them, then revalidate your installation via Steam (= right click on the game your library, then "Properties").

If that does not help, please place a ticket with tech support through https://support.sega.co.uk/ - they may be able to help you figure it out.